This invention relates generally to methods and apparatus for bonding optical fibers together. More particularly, this invention relates to methods and apparatus for forming fiber optic couplers. Still more particularly, this invention relates to methods and apparatus for forming a coherent molecular bond between two optical fibers.
Some familiarity with the propagation characteristics of light within an optical fiber will facilitate an understanding of both the present invention and the prior art. Therefore, a brief discussion of fiber optic waveguides, normal modes of propagation of light in such waveguides and polarization of light is presented.
The behavior of an optical wave at an interface between two dielectric materials depends upon the refractive indices of the two materials. If the refractive indices of the two dielectrics are identical, then the wave propagates across the interface without experiencing any change. In the general case of different refractive indices, however, there will be a reflected wave, which remains in the medium in which the wave was first propagating, and a refracted wave, which propagates beyond the dielectric interface into the second material with a change in direction relative to the incident wave. The relative intensities of the reflected and refracted waves depend upon the angle of incidence and the difference between the refractive indices of the two materials. If an optical wave originally propagating in the higher index material strikes the interface at an angle of incidence greater than or equal to a critical angle, there will be no refracted wave propagated across the interface; and essentially all of the wave will be totally internally reflected back into the high index region. An exponentially decaying evanescent wave associated with the incident wave extends a small distance beyond the interface.
Optical fiber has an elongated generally cylindrical core of higher refractive index and a cladding of lower refractive index surrounding the core. Optical fiber use the principle of total internal reflection to confine the energy associated with an optical wave to the core. The diameter of the core is so small that a light beam propagating in the core strikes the core only at angles greater than the critical angle. Therefore, a light beam follows an essentially zig-zag path in the core as it moves between points on the core-cladding interface.
It is well-known that a light wave may be represented by a time-varying electromagnetic field comprising orthogonal electric and magnetic field vectors having a frequency equal to the frequency of the light wave. An electromagnetic wave propagating through a guiding structure can be described by a set of normal modes. The normal modes are the permissible distributions of the electric and magnetic fields within the guiding structure, for example, a fiber optic waveguide. The field distributions are directly related to the distribution of energy within the structure. The normal modes are generally represented by mathematical functions that describe the field components in the wave in terms of the frequency and spatial distribution in the guiding structure. The specific functions that describe the normal modes of a waveguide depend upon the geometry of the waveguide. For an optical fiber, where the guided wave is confined to a structure havinf a circular cross section of fixed dimensions, only fields having certain frequencies and spatial distributions will propagate without severe attenuation. The waves having field components that propagate unattenuated are the normal modes. A single mode fiber will guide only one energy distribution, and a multimode fiber will simultaneously guide a plurality of energy distributions. The primary characteristic that determines the number of modes a fiber will guide is the ratio of the diameter of the fiber core to the wavelength of the light propagated by the fiber.
In describing the normal modes, it is convenient to refer to the direction of the electric and magnetic fields relative to the direction of propagation of the wave. If only the electric field vector is perpendicular to the direction of propagation, which is usually called the optic axis, then the wave is said to be a transverse electric (TE) mode. If only the magnetic field vector is perpendicular to to the optic axis, the wave is a transverse magnetic (TM) mode. If both the electric and magnetic field vectors are perpendicular to the optic axis, then the wave is a transverse electromagnetic (TEM) mode. None of the normal modes require a definite direction of the field components: and in a TE mode, for example, the electric field may be in any direction that is perpendicular to the optic axis.
The direction of the electric field vector in an electromagnetic wave is the polarization of the wave. In general, a wave will have random polarization in which there is a uniform distribution of electric field vectors pointing in all directions permissible for each mode. If all the electric field vectors in a wave point in only one particular direction, the wave is linearly polarized. If the electric field consists of two orthogonal electric field components of equal magnitude and 45.degree. out of phase, the electric field is circularly polarized because the net electric field is then a vector that rotates around the optic axis at an angular velocity equal to the frequency of the wave. If the two linear polarizations have unequal magnitudes and phases that are neither equal nor opposite, the wave has elliptical polarization. In general, any arbitray polarization can be represented by either the sum of two orthogonal linear polarizations, two oppositely directed circular polarizations or two oppositely directed elliptical polarizations having orthogonal semi-major axes.
Propagation characteristics such as velocity, for example, of an optical wave depend upon the polarization of the wave and the index of refraction of the medium through which the light propagates. Certain materials, including optical fiber, have different refractive indices for different polarizations. A material that has two refractive indices is said to be birefringent.
The polarization of an optical signal is sometimes referred to as a mode. A standard single mode optical fiber will propagate two waves of the same frequency and spatial distribution that have two different polarizations. A multimode fiber will propagate two polarizations for each propagation mode. Two different polarization components of the same normal mode can propagate through a birefringent material unchanged except for a difference in velocity of the two polarizations. Polarization is particularly important in interferometric sensors because only waves having the same polarization will produce the desired interference patterns.
An optical coupler joins two fibers for transmitting optical energy from one fiber to the other. Optical couplers are used in many applications of optical fiber, including constructing fiber optic interferometers, resonators, sensor arrays and data buses.
Among the parameters that are considered in forming an optical coupler are the polarizations of the waves before and after coupling, the fraction of energy to be coupled from one fiber to the other, the insertion loss of the coupler and whether the fibers guide only a single mode or a multiplicity of modes.
Several methods have been employed for joining two fibers to form a coupler for transmitting optical energy from one fiber to the other. A first technique for joining two fibers results in the biconical tapered fiber optic coupler. The fibers are placed together, twisted and then heated to near the melting point to fuse them during application of a force to stretch the fibers. The heating and twisting steps in the formation of the biconical tapered fiber optic coupler result in alteration of the molecular arrangements of the materials comprising the joined fibers. In particular, the heating and twisting alters the distributions of impurities used as dopants to control the refractive indices of the core.
The refractive index of a material depends upon its molecular structure; therefore, forming the biconical tapered fiber optic coupler, in general, produces localized changes in the refractive indices of the fibers. These changes in refractive indices are uncontrollable and cause undesirable, uncontrollable reflections and refraction at the interfaces between the fibers. It is therefore difficult to fabricate biconical tapered fiber optic coupler having coupling efficiencies predetermined for specific applications.
Another technique for joining two fibers includes grinding flat surfaces on facing portions of the fibers and joining them by mechanically fixing the surfaces in juxtaposition with a refractive index matching oil or other material that enhances light transmission between the fibers.
These and other prior methods of forming fiber optic couplers have the disadvantage of suffering undesirably high loss of optical signal intensity. These prior fabrication techniques generally fail to facilitate control of the amount of light transmitted by any particular coupler and also fail to permit control of the amount of light that will be coupled from one fiber to another. Such problems are caused by the inherent intermolecular inhomogeneities and discontinuities that occur at the interfaces during prior fabrication processes for joining fibers. Further, prior fabrication techniques are not conducive to producing the great numbers of optical couplers required in communications, data processing and sensor applications.
Many applications of fiber optic couplers require very low signal loss in the coupler and also require couplers having critically selected light transmission and coupling characteristics. Large numbers of such couplers are required for practical applications of optical fibers.